In case of known agitator ball mills of this type there is the serious problem which is still not solved satisfactorily that the sieve arranged upstream of the milling material outlet often clogs in an increasing manner after a short operating time and thus decreases the throughput until finally an interruption of the operation is necessary in order to clean the sieve.
In order to solve this problem, various constructional solutions involving separators arranged upstream of the sieve have been proposed. One of the most recent developments in this direction is described e.g. in EP-751830. This development provides to arrange, in addition to a separator arranged upstream of the sieve and upstream of said separator, an upstream device for pre-screening between coarse and fine particles. This device substantially provides to make the distance between the last disk of the agitator and the separator small enough that the mixture of milling material and milling media in this intermediate space is stronger accelerated than in the remaining milling chamber and thereby, the separation, which is called “pre-screening”, takes place. In this manner it is to be achieved that less or no coarse milling media reach the sieve.
However, it is most likely that in a continuous operation, even with this solution, clogging of the sieve can take place. It is therefore the object of the invention to prevent clogging of the sieve in a different and more effective manner.